REMOVE THE IMPLANTS

Instead, allow players to fit implants from slots 6-10 into these slots.
HOWEVER, you CANNOT stack them. So you can't fit 2 - 5% heavy missile damage implants.

This would allow players to implant for 2 ships instead of 1, or can mix and match implants that will help fitting AND damage of active ship.

As far as pirate implants, allow them to keep their current bonus; However, to replace the attribute bonus, allow them to give an additional % bonus in the co-slot.
IE, if you have a 5% heavy missile implant in slot 7, putting a pirate implant, in slot 2, will give you the standard bonus plus an additional 1-2% buff to heavy missiles.

At 4 years, such a +5 character has maybe gained 4-5 months over a character that only uses +3 implants all the time. (1 month per 408 days of this plan, no remap from the standard)

With another plan that is a bit more closer to what people might train, such a person also gains barely a month over a character with +3s.

Now, there is a difference, obviously; however, it is by far not as large as people depict it. It takes ~12 years (maybe 10 if I factor in remaps) of sitting in a +5 to gain a full year over a +3 character.
If a person sits this long in a +5, just to get the skills a couple of days per month, a month per year, a couple of months per half a decade faster, by all means let them ruin their game. If these people complain about how boring the game is, by all means show them their failure. If a person in +3s is complaining about how slow skill training is, suggest them to use +5s but also visualize to them what they risk if they continue their current path of action (maybe that person was already active in low sec and is merrily shooting stuff) and/or what they are going to lose in terms of action/activity/content if they confine themselves to High sec for just a couple of days difference.

I am now 4 years old, and I have reached a point where I do nothing more but hone what I already know in terms of skills and can do nothing else but to branch out into other races (I used to be quite Amarr focused), but now I have trained almost everything that Amarr has to offer. This is 6 more years of training until he has such a lead over me. And by then, it does not matter at all anymore if I can fly 3 carriers and he 4.

The potential comes with more risk. If you undock in that pod and die in High sec to a ganker, which is more likely these days than ever before, you lost hundreds of millions. By keeping these implants safe in a station, however, you also lose out on a lot of action and make your game experience less enjoyable (depending on your needs and ambitions). There are more risks to the game than just losing stuff. Which means, by forcing yourself to a fast training, you consciously bar yourself from the game itself and confine yourself into an unsatisfying existence.